The present invention relates to an apparatus and method for conveying information from a variety of sources to a variety of destinations, via a common network. More particularly, the invention relates to an apparatus and method for mapping pulse-coded modulated signals into packets and switching the packets in a network.
A typical digital telecommunications network provides a continuous bit rate service using Time Division Multiplexing (TDM). Telephone sets as well as other terminal devices are connected to network ports via telephone lines. The network ports include interfaces for converting analog signals from the terminal devices into pulse code modulated (PCM) signals for transmission through the digital telecommunications network. In a communication between an originating port and a destination port on the digital network, information is transmitted over a single high-speed channel in a pre-assigned time-slot on periodic transmit and receive frames. Circuit switches operate to switch the information from the originating port to the destination port.
While TDM networks provide an adequate service for synchronous data like voice and video, TDM is not well suited for bursty (i.e., asynchronous) computer transmissions. As a result, packet switching was introduced to provide efficient transport of computer transmissions. In packet switching networks, data signals are arranged into packets of any convenient length. The packets can be a fixed-length or a variable length. Each packet includes a header for, among other things, specifying a destination of the packet. After a packet has been assembled, a high-speed transmission path is allocated, but only for a time sufficient to transport the packet of data toward its destination. Although digitized voice can be transported in this manner, the wide variances in delay caused by the operating characteristics of a packet network has demonstrated that packet switching is less than desirable at this point in time.
Fortunately, a broadband communications standard exists for accommodating both synchronous and asynchronous communication applications. This standard, known as the Asynchronous Transfer Mode (ATM) standard, packs data into frames, each frame comprising a plurality of “cells,” each cell being 53 bytes (i.e., octets) long. The 53 bytes in an ATM cell include a 5-byte header and a 48-byte payload. The 5-byte header generally includes a virtual path identifier (VPI) portion to associate the cell with a virtual path, a virtual channel identifier (VCI) portion to associate the cell with a virtual channel, a payload type portion to identify the type of information in the payload, a header error control portion, and a group flow control portion. The recommended standards are defined by the ATM Forum and are available from several publishers, such as Prentice Hall of Englewood Cliffs, N.J. 07632, under the title ATM User-Network Interface Specification Version 3.0 (ISBN 0-13-225863-3).
The ATM protocol was designed to support many different applications within a network, and to treat each application according to its needs. Specifically, the ATM protocol allows voice, video, and computer transmissions to be combined over the same network. As noted, the transmission needs of each of these applications vary. In particular, some forms of data, like e-mail, are delay insensitive while other forms of data, like voice and video, are delay sensitive. To meet the transmission needs of each application, ATM networks use complicated rules that specify how the network should treat each information type. In particular, typical ATM networks provide a set of instructions for packing information into the ATM cells.
The process of packing information signals into cells or packets is known as “mapping.” For each information type supported by the network, the network implements a different mapping scheme dependent upon the transmission needs of that information type. In addition to the mapping scheme, the network assigns a priority level to each information type. The network then allocates more resources to higher priority data. For example, when there is more information that must be transmitted in a given frame than there are available cells for carrying that information, higher priority level data will be transmitted first. In this manner, the various information signals may or may not be granted access to the network.
The complexity of the ATM mapping often leads to lengthy delays. Real time data, such as video and voice, may be adversely affected by such delays. For example, delays in the delivery of voice signals may cause echoing and jitter, thereby hindering the natural flow of conversation. In most ATM systems, multiple samples of a single audio stream are collected until a fixed-size ATM cell is full; this directly causes a six-millisecond delay. In a typical long-distance communication, there are repeated conversions from ATM to TDM and from TDM to ATM to make use of existing public switching telephone network (PSTN) facilities (which use T1/E1 lines). Each such conversion into ATM cells causes the six-millisecond packetization delay. End to end, such delays can easily exceed the level where echo cancellation is required for analog telephone sets. In addition, echo cancellation is difficult and often imperfect.
In some ATM systems, samples from various data streams are collected and placed into each fixed-size ATM cell. In other words, samples from the same data stream may be transmitted in different ATM cells. These ATM cells may be routed through different paths to the destination. As a result, samples from the same data stream might not be received at the destination end in the same order in which they were sent at the originating end. The network includes components to reorder the information signals. This reordering of the data may result in additional delay.
ATM switches may introduce further delay in the transmission of data. ATM switches receive incoming ATM cells on a virtual connection and switch the entire cell to another virtual connection based on destination information in the cell's header. The ATM switches often establish a new route for each cell that they switch. The establishment of the virtual connections on an as-needed basis may introduce network control delays.